ABSTRACT Twenty-five pairs of North American beavers Castor canadensis Kuhl were introduced to Tierra del Fuego Island in 1946. The population has expanded across the archipelago, arriving at the Chilean mainland by the mid-1990s. Densities range principally between 0.5-2.05 colonies/km. They have an impact on between 30-50% of stream length and occupy 2-15% of landscape area with impoundments and meadows. Beaver impacts constitute the largest landscape-level alteration in subantarctic forests since the last ice age. 2. The colonization pattern, colony densities and impacted area indicate that habitat in the austral archipelago is optimal for beaver invasion, due to low predator pressure and suitable food resources. Nothofagus pumilio forests are particularly appropriate habitat, but a more recent invasion is occurring in adjacent steppe ecosystems. Nonetheless, Nothofagus repro- ductive strategies are not well adapted to sustain high beaver population levels. 3. Our assessment shows that at the patch-scale in stream and riparian ecosystems, the direction and magnitude of exotic beaver impacts are predictable from expectations derived from North American studies, relating ecosystem engineering with underlying ecological mechanisms such as the relationships of habitat heterogeneity and productivity on species richness and ecosystem function. 4. Based on data from the species' native and exotic range, our ability to predict the effects of beavers is based on: (i) understanding the ecological relationships of its engineering effects on habitat, trophic dynamics and disturbance regimes, and (ii) having an adequate compre- hension of the landscape context and natural history of the ecosystem being engineered. 5. We conclude that beaver eradication strategies and subsequent ecosystem restoration efforts, currently being considered in southern Chile and Argentina, should focus on the ecology of native ecosystems rather than the biology of this invasive species per se. Further- more, given the nature of the subantarctic landscape, streams will probably respond to restoration efforts more quickly than riparian ecosystems.

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_______________________________________________________________________________________ SUMMARY In 2007, a field visit by members of the International Mire Conservation Group (IMCG) to the Atlantic coast of Peninsula Mitre (the easternmost part of Isla Grande de Tierra del Fuego, Argentina) gathered information on mire diversity in this remote wild area with largely pristine mires. Our expedition showed that Peninsula Mitre hosts a wide variety of habitats across two exciting ecological gradients: (i) a regional west–east gradient from Sphagnum magellanicum dominated mires in the west to Astelia pumila dominated mires in the east; and (ii) a gradient from extremely acid to extremely carbonate rich mire types induced by local bedrock. The large variety of hydromorphological mire types comprises raised bogs, blanket bogs, sloping fens, string fens, flat fens and calcareous spring fens. In the Atlantic coastal area, the abundance of Sphagnum magellanicum in the ombrogenic systems decreases conspicuously from west to east with the species being almost absent in the east. However, the fossil record shows thick layers of Sphagnum peat close beneath mire surfaces everywhere, indicating that substantial hydrological and ecological changes have taken place in the recent past. We observed large scale erosion in the mires along the Atlantic coast. Locally, well-developed fen systems are present, including calcareous spring fens with active travertine (tufa) deposition. The regional vegetation can be regarded as a parallel to that of boreal oceanic regions in the northern hemisphere. The mires and peatlands of the peninsula are of global significance. They are impressive, peculiar, extensive and largely pristine mires in a globally very rare climatic and biogeographical context embedded in a landscape with significant natural dynamics. The damaging impact of free-roaming cattle on the mires and upland vegetation is, however, conspicuous and needs urgent attention. Peninsula Mitre deserves the highest possible protection, e.g. as a provincial protected area and a World Heritage Site.

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